The present invention relates to a device for removing developer liquid from a recording material present in an electrophotographic copying machine in which the recording material is charged and exposed to an optical image of an original, and the latent charge image obtained is developed into a visible image by applying developer liquid. For the purpose of applying the developer liquid, a roller rotating in the direction opposite to the direction of movement of the recording material is used, and the roller is arranged in such a way that a gap is produced between the roller and the recording material.
A device of this type is described in U.S. Pat. No. 3,907,423. In this device, a squeezing roller for removing the developer liquid rotates in a collecting vessel and thereby removes developer liquid from the photoconductor layer. The smaller the gap is between the photoconductor layer and the counter-rotating roller, the better is the stripping effect. In this device, the width of the gap is between about 0.05 and 1 mm. The roller is electrically insulated from the copier, and a bias voltage is applied to it. In order to make sure that the gap is always maintained, support rollers are provided at the side of the squeezing roller, which rolls with the aid of said support rollers.
By means of a doctor appliance which is firmly disposed in close proximity to the roller, excess developer liquid is removed and the roller is cleaned. The developer liquid stripped off by the doctor appliance is collected in the collecting vessel.
It is necessary to remove the excess developer liquid in order to ensure that only a developer liquid layer having an exactly defined thickness remains on the surface of the recording material and to facilitate cleaning of the photoconductor layer before a new copying cycle is started. By reducing the thickness of the developer liquid layer present on the photoconductor layer prior to transmitting the developed charge image, the quality of the transmitted charge image is improved, and the copies can be dried more easily.
In the device known from German Offenlegungsschrift No. 19 37 019, a squeezing roller is pressed against the surface of a recording material, whereby excess developer liquid is squeezed off. This process involves the danger, however, of damaging the toner image.
In the device known from U.S. Pat. No. 3,596,635, too, a roller touching the surface of the recording material is provided, by means of which excess developer liquid is removed from the surface of the recording material. A problem arising with all of these known devices is, however, that the pressure between the squeezing roller and the surface of the recording material may vary considerably, since it is very difficult to produce rollers having perfectly even and uniformly curved circumferential surfaces. Photoconductor drums, to the circumferential surfaces of which the recording materials are applied, in general have bulging or barrel-shaped deformations, whereby deviations from an ideal cylindrical surface of up to 0.02 mm may occur, and in rare cases these deviations are even higher. The diameters of squeezing rollers are also subjected to certain manufacturing variations due to the after-treatment of their surfaces, so that, in cases where the squeezing rollers are in direct contact with a photoconductor drum, it must be taken into account that there are differences in pressure between the two rollers, which may lead to an irregular transmission of the toner images from the photoconductor drum to the image-receiving material.
The squeezing rollers conventionally employed in electrophotographic copying machines usually have an aluminum surface which is provided with an anodized coating. It has been shown that, when the squeezing roller has been in use for a longer time, fine grooves form even in the anodized layer due to dust settling on the doctor. These grooves may negatively influence the stripping-off by means of the doctor of the excess developer liquid squeezed off the photoconductor drum.
An effort must therefore be made to maintain as small as possible the gap between the photoconductor drum, or, respectively, the recording material, and the squeezing roller for removing the excess developer liquid. It is not possible, however, to realize this aim of reducing the gap width, and at the same time always and at each point maintain a gap, merely by changing the diameter of the support rollers on which the squeezing roller rolls or by changing the diameter of the squeezing roller. This is because of the abovementioned irregularities in the curvature of the cylindrical surfaces occurring during the manufacturing process.